Function make_error_str

system/lib/libcxx/src/system_error.cpp:210–218

#endif // _LIBCPP_HAS_THREADS

string make_error_str(const error_code& ec, string what_arg) {
  if (ec) {
    if (!what_arg.empty()) {
      what_arg += ": ";
    }
    what_arg += ec.message();
  }
  return what_arg;
}

string make_error_str(const error_code& ec) {
  if (ec) {